(1) Field of the Invention
The present invention relates to plethysmographs used to measure changes in air volume, such as in pulmonary testing of small animals, and in particular to plethysmographs facilitating accurate positioning of anesthetized animals for tracheal cannula insertion during laboratory testing.
(2) Description of the Prior Art
Various laboratory procedures require an analysis of the respiratory patterns of small test animals. Respiratory data is collected by enclosing the animal in the chamber of a device known as a plethysmograph, and measuring changes in air pressure within the chamber during the test procedure.
A plethysmograph is generally comprised of an enclosure with a closeable access opening to permit placement and removal of the test animal, and a cover to releasibly fit over the enclosure access opening. The enclosure is provided with a means for controlled air access and egress, a means to measure variations in air pressure within the enclosure, such as a differential pressure transducer, and commonly a means for administering test materials to the animal during the test procedure.
Test animals may be anesthetized and tracheotomized during many test procedures. Therefore, plethysmographs often include a table to support the animal. Usually, the table will include a horizontal upper surface upon which the animal is placed, normally on its back, with the animal's head toward the end or side of the chamber through which the tracheal cannula is inserted during the test procedure.
To facilitate opening and closing, one section of the enclosure, either the open-ended chamber or the cover, may be held in a stationary position, while the other section is moveable between an open position and a closed position. In the closed position, the open end of the chamber releasibly engages the cover in a sealing fashion to prevent air from entering the chamber between the two sections. For example, as taught in U.S. Pat. No. 5,379,778, an air-tight seal may be achieved by positioning an O-ring between the chamber opening and the cover.
Prior art plethysmographs are less than optimal. First, due to variations in animal sizes, accurate placement of the tracheal cannula is not always possible. Second, fitting of the chamber cover onto the chamber may be difficult due to the need to align the chamber opening with the corresponding connection area of the cover. Therefore, there is a need for further improvements in prior art plethysmographs.